tempus fugit

An article in Nature describes new information on the shape of the ground underneath the Antarctic ice sheets that suggests that the sheets could melt much more quickly than previously estimated, contributing 64 to 114 centimeters to the rising sea and resulting in 1.5 to 2.1 meters (5-6 feet) total of sea level rise by 2100. This is also because of “previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs.”

Earlier estimates indicated a rise of one to four feet by 2100, according to the United States Global Change Research Program (USGCRP) National Climate Assessment projections in 2014. Globally, sea levels are already rising faster than expected: according to Wikipedia, 2.6-2.9 mm/year since 1993. Wikipedia says that since 1870, global sea levels have risen 195 millimetres (7.7 in), an average of 1.7 millimetres a year over the entire period. Increasing rates of rise have been seen recently: satellite observations suggest a rise of 3.3 mm a year since 1993.

Uncertainties in drawing ancient coastlines contribute to uncertainty in predicting future sea level rise because tectonic activity has altered the height of the continents over millions of years. These changes have contributed to a potential variation of “35 centimeters or more” in predictions for 2100 according to a Science News article reporting the new Antarctic findings.

Continental uplift caused by groundwater depletion in such areas as California and southern Asia has counteracted some of this sea level rise locally: another research article published online on April 2 describes a rebound of about 25 mm or 0.4 mm a year during the last hundred years. While this will protect the West Coast of the US, it will have little effect on global sea levels.

In the Southern Pacific Ocean, the situation is different. “Changing global trade winds have raised sea levels in the South Pacific about a foot over the past 30 years” (from an article in the NYT last December.) With most of the Marshall Islands less than six feet above sea level now, they are expecting to be overtopped within 50 years by the rising South Pacific. Already, farming on these islands is being wiped out because of salt water infiltrating the ground water table.

This is economically important to the United States due to treaty obligations: the residents of the Marshall Islands have been given legal permission to immigrate to the US because they were displaced by the detonation of 67 nuclear weapons on and over the Bikini Atoll by the US and France. 700,000 people have the right to immigrate to the US because they were moved by the Army as a precaution against fallout before the bombs were set off.

On the top of the world, Greenland’s ice sheet is in trouble too: ” The full melting of Greenland’s ice sheet could increase sea levels by about 20 feet.” (from another NYT article, last October, with the ominous title “Greenland is Melting Away.”) A Scientific American article (reprinted from ClimateWire) says that Greenland’s ice cap and glaciers show melting much faster than the rest of Greenland’s ice cover, contributing about ten percent of the whole world’s ice loss. The article says that the ice cap and glaciers are only about 5-7 percent of Greenland’s total ice. It also says that the southeast is melting faster than the northern part of Greenland despite its heavy snowfall. Glaciers in the southeast often feed right into the sea, sending icebergs into shipping lanes (this was the source of the iceberg that sank the Titanic.)